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This circular is intended to answer questions asked by correspondents regarding the progress and character of the work of the "Irrigation Survey" and of related investigations being carried on by the Division of Hydrography of the United States Geological Survey. It also gives a review of the legislation authorizing this work, together with a list of publications of the Geological Survey showing the results accomplished.
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Description of instruments.-The temperature and humidity of the air are obtained from the simultaneous observation of a pair of mercurial thermometers termed the dry and the wet bulb. The air temperature is given by the dry-bulb thermometer, and the humidity is obtained from the combined readings of both. The wet-bulb thermometer differs from the dry-bulb thermometer only in having its bulb covered with thin muslin, which is wetted in pure water at each observation.The two thermometers are fastened in a light metal 'or wooden frame. To this frame is to be attached a stout cord for the whirling of the thermometers, which is an essential part of every observation.
The ·rate of evaporation from water surfaces varies with the temperature of the water, the velocity of the wind at the water surface, and the dryness of the air. Consequently, the rate of evaporation from rivers, lakes, canals, or reservoirs varies widely in different localities and for the same locality in different seasons.
In the prosecution of the general "survey of the arid lands for purposes of irrigation," authorized by Congress to be undertaken by the U. S. Geological Survey, a determination of the amount of water supplied by the natural rain and snow fall in different localities is of fundamental importance. To obtain this knowledge the Geological Survey must depend in large measure upon the residents, to whom the benefit of the work will accrue, for their voluntary cooperation in making; the necessary observations.